1. Field of the Invention
The present invention relates to a diamond single crystal composite substrate and a method for manufacturing the same, and more particularly relates to a high-quality diamond single crystal composite substrate with a large area that can be used in semiconductor materials, electronic parts, optical parts and the like, and a method for manufacturing the same.
2. Description of the Prior Art
As a semiconductor material, diamond has numerous superior characteristics not found in other materials, such as a high thermal conductivity, a high electron/positive hole mobility, a high dielectric breakdown field, a low dielectric loss, a broad band gap and the like. Especially in recent years, ultraviolet light-emitting devices utilizing this broad band gap, field effect transistors with superior high-frequency characteristics and the like have been developed.
As in the case of other semiconductor materials, a high-quality single crystal substrate is needed in order to utilize diamond as a semiconductor. Currently, diamond single crystals that are obtained industrially mainly by high-temperature high-pressure synthesis methods are superior in terms of crystallinity even compared to naturally produced single crystals, and can be utilized as semiconductor substrates from the standpoint of physical properties; however, the size of the single crystals obtained is limited to around 1 cm. In the case of such small substrates, semiconductor wafer processes utilizing steppers, electron beam exposure or the like generally use in the micro-fabrication or the like of Si or the like become a problem. In the case of small substrates, it is difficult to use such working devices that were designed for use with wafers having a diameter of several inches, and even if working devices especially meant for use with small substrates are introduced, the difficulty of peripheral processes such as the photo-resist coating process and the like is not resolved.
Accordingly, for example, a method for obtaining large integrated single crystals by disposing a plurality of high pressure phase substances that have substantially the same crystal orientations, thus forming substrates that act as nuclei for vapor phase growth, and then growing a single crystal on top of these nuclei, has been proposed as a method for obtaining diamond single crystal substrates that have a large area (see Japanese Patent Publication No. 3-75298 A).
When the method for obtaining large single crystals according to Japanese Patent Publication No. 3-75298 A is used, the single crystal substrates comprising a plurality of substrates that are used as nuclei for vapor phase growth ordinarily do not have growth plane orientations that are completely identical, but instead have plane orientations that are slightly different from each other. When a single crystal is integrated by performing single crystal vapor phase growth from here, the connecting parts have growth boundaries with different angles, i. e., defects in the broad sense, that are called small angle boundaries, and these grain boundaries do not substantially disappear even if crystal growth is continued. Accordingly, the semiconductor physical properties of region straddling such small angle boundaries are inferior to those of a complete single crystal, and if a device or the like is manufactured on top of the integrated single crystal, the performance is lost in the areas that straddle the small angle boundaries.